The present invention is concerned in general with circuit arrangements which alter the dynamic range of audio signals, namely compressors which compress the dynamic range and expanders which expand the dynamic range. More particularly, the invention relates to improvements in transient control aspects of circuit arrangements for altering the dynamic range of audio signals.
Compressors and expanders are normally used together (a compander system) to effect noise reduction; the signal is compressed before transmission or recording and expanded after reception or playback from the transmission channel. However, compressors may be used alone to reduce the dynamic range, e.g., to suit the capacity of a transmission channel, without subsequent expansion when the compressed signal is adequate for the end purpose. In addition, compressors alone are used in certain products, especially audio products which are intended only to transmit or record compressed broadcast or pre-recorded signals. Expanders alone are used in certain products, especially audio products which are intended only to receive or play back already compressed broadcast or pre-recorded signals. In certain products, a single device is often configured for switchable mode operation as a compressor to record signals and as an expander to play back compressed broadcast or pre-recorded signals.
A dominant signal component is a signal component having a substantial enough level so as to effect dynamic action within the frequency band under consideration. Under complex signal conditions there may be more than one dominant signal component or a dominant signal component and sub-dominant signal components. In a compander system which relies on complementarity of the compressor and expander, all of the signal components must be compressed and expanded in accordance with a defined compression/expansion law in order that the signal spectrum including the dominant signal component (and other signals affected by dynamic action) can be restored to their correct levels in the expander.
Sliding band circuits employ signal dependent variable filtering to provide dynamic action. Generally, a dominant signal component causes the cutoff or turnover frequency (or frequencies) of one or more variable filters (e.g., high pass, low pass, shelf, notch, etc.) to shift so as to compress or expand the dominant signal component. For example, for the case of high frequency audio compression or expansion a high frequency boost (for compression) or cut (for expansion) can be achieved by using a high pass filter with a variable lower corner frequency. As the signal in the high frequency band increases, the filter corner frequency slides upwardly so as to narrow the boosted or cut band and exclude the useful signal from the boost or cut. Such circuits can also be configured to act at low frequencies in which case low frequency boost or cut can be provided by way of a low pass filter with a variable upper corner frequency.
A sliding band system operating only in a single high frequency band is described in U.S. Pat. No. Re. 28,426 and U.S. Pat. No. 4,490,691. This system, which forms the basis for the well known consumer companding type audio noise reduction system known as B-type noise reduction, includes, in a dual path arrangement, a side path having a fixed high pass filter in series with a variable filter.
Fixed band circuits employ variable gain or loss devices to provide dynamic action. In fixed band circuits the compression or expansion is effected to the same degree within the entire frequency band in which the circuit operates.
Examples of fixed band circuits are to be found in U.S. Pat. No. 3,846,719; 3,903,485; and in Journal of the Audio Engineering Society, Vol. 15, No. 4, October, 1967, pp. 383-388. In the latter reference, the well known professional companding type audio noise reduction system known as A-type noise reduction is described. In that system, fixed band circuits are embodied in bandsplitting arrangements in which the frequency spectrum is split into a plurality of bands by corresponding bandpass filters and the dynamic action is essentially independent in each frequency band.
It is also possible to employ a single fixed band circuit operating throughout the input signal frequency band. Such arrangements are known as wideband compressors and expanders.
A "dual path" arrangement is one in which a compression or expansion characteristic is achieved through the use of a main path which is essentially free of dynamic action and one or more secondary or side paths having dynamic action. The side path or paths take their input from the input or output of the main path and their output or outputs are additively or subtractively combined with the main path in order to provide compression or expansion. Generally, a side path provides a type of limiting or variable attenuation (such as by way of a fixed band or sliding band circuit) and the manner in which it is connected to the main path determines if it boosts (to provide compression) or bucks (opposes) (to provide expansion) the main path signal components. Such dual path arrangements are described in detail in U.S. Pat. No. 3,846,719; 3,903,485; 4,490,691 and U.S. Pat. No. Re. 28,426.
Although it is possible to configure a sliding band circuit or a fixed band circuit using as the variable element an automatically responsive device, such as a diode type of limiting device, it is generally preferred to employ a controlled device that is responsive to a control signal The latter approach gives the circuit designer flexibility in controlling the operation of the circuit by performing operations on the control signal (for example, frequency selective and/or level dependent amplification of the control signal as done in the A-type and B-type systems).
In the A-type and B-type systems mentioned above, the source-drain path of field effect transistors (FETs) are employed as voltage controlled variable resistors (forming the variable element of a variable attenuator in the A-type system and forming the variable element of a variable filter in the B-type system). DC control voltages, derived from the input signals, are applied to the FET gates. The derivation includes rectification, smoothing, and adjustment of the control voltage amplitude as necessary to achieve the desired dynamic action. As the control voltage increases, the degree of limiting increases: by increasing the attenuation in the fixed band circuits and, in the sliding band circuits, by shifting the corner frequency of the filter farther and farther from its quiescent position.
One disadvantage of the control circuit arrangement in the A-type, B-type, and other known compander systems is that the DC control signal is formed from the linear additive combination of the pass-band signals and the stop-band signals reaching the control circuit. In the case of fixed band circuits in a bandsplitting system, the pass-band is the frequency band in which a particular circuit operates; the stop-band is the remainder of the signal spectrum handled by the system. In the case of sliding band circuits, the pass-band is the frequency band within the pass-band of the variable filter and the stop-band is the frequency band outside its pass-band. In an ideal circuit, compression or expansion should not be affected by the levels of signals outside the pass-band of the fixed band or the pass-band of the sliding band (whether or not in its quiescent position). A solution to the problem is set forth in U.S. Pat. No. 4,498,055.
In accordance with the teachings in U.S. Pat. No. 4,498,055, the formation of the DC control signal is altered, in a level dependent way, so as to make the DC control signal less responsive to stop-band signal components as the level of the input signal rises. In practical embodiments, this is accomplished by opposing (or bucking) the control signal with a signal referred to as the modulation control signal. In the case of a fixed band circuit, the modulation control signal assures that the amount of gain becomes no more than necessary to assure that a dominant controlling signal is not boosted (in the case of compression) above a reference level. In the case of a sliding band circuit, the modulation control signal assures that the amount of frequency sliding of the variable filter is no more than necessary to assure that a dominant controlling signal is not boosted (in the case of compression) above a reference level. In each case, the modulation control signal causes the DC control signal to be less than it would be otherwise for high level signals. As a consequence, for high level input signals, the output signals from the dynamic action circuit are higher than they would be otherwise.
A basic design conflict in companding type noise reduction systems is the requirement to balance the ability to handle rapidly changing waveforms (to minimize signal overshoots) against the desirability of minimizing signal modulation and noise modulation. The ability of a compressor (or expander) to respond to a rapid amplitude change in its input signal is directly related to its attack time or the time which is required for the device to change its gain (or shift its filter corner frequency) in response to the input amplitude change. Long attack times tend to reduce modulation distortion. When the change in input signal amplitude occurs more abruptly than the device is capable of changing its gain or corner frequency (caused by control circuit lag), an overshoot results. For example, if a compressor has a gain of two times (resulting from some steady state input condition) and suddenly the input signal doubles in amplitude such that that compressor is unable to reduce its gain to provide the desired gain according to its compression law, the output signal will exceed its desired amplitude and may exceed the desired maximum output of the device, depending on the amplitude jump and suddenness with which the input signal increases. Such an increase in output is referred to as an overshoot. Overshoots normally have maximum amplitudes equal in value to the degree of compression. The overshoot will continue until the input signal is suitably changed or, if the input signal remains constant at its new high level, until the control circuit time lag is sufficiently overcome so as to reduce the gain of the compressor to the gain directed by its compression law. Overshoots are undesirable because they can overload the channel or device carrying the output signal from the compressor.
Various companding systems have approached the problem of overshoots in different ways: fixed attack times, using a short attack time or a long attack time, and, variable attack times. A short attack time tends to minimize the amplitude and time of the overshoot but has an undesirable side effect in that rapid changes in gain cause significant modulation products to be generated. In order for such modulation products generated in a compressor to be cancelled, the compressed signal must be carried by a linear phase channel and the expander must provide reciprocal treatment. Such requirements are difficult or impossible to meet in practical situations. A long attack time has the advantage that modulation products are minimized but significant overshoots are produced. Accordingly, some companding systems have employed arrangements in which the attack time is variable, remaining relatively long during steady-state signal conditions but changing to a short attack time during input transients.
In the A-type system mentioned above, variable attack time control circuits are employed. In addition, the system is a dual-path arrangement that takes advantage of the fact that the side paths, in which the limiting takes place, should not have signals exceeding a predictable maximum amplitude. Accordingly, non-linear clipping is employed in the side paths as a back-up or secondary overshoot suppression in addition to the variable shortening of the control signal attack time. Because the signal level in the side paths are substantially less than that in the main path, the distortion introduced is small. In addition the non-linear clipping acts very briefly and infrequently.
As mentioned above, the introduction of the modulation control technique results in a larger output signal than would occur without modulation control. If modulation control were incorporated into a dual-path system, such as the A-type system, the signal in the side path would be larger than otherwise and would not have a predictable maximum amplitude: the side path signal would continue to rise with the input signal. Accordingly, a back-up overshoot suppression arrangement in the side path employing non-linear clipping at a fixed level would not be operable if modulation control were used.
The variable attack time control circuits used in the A-type system employ first and second integrators (smoothing circuits) coupled by components that include "speed up" diodes that function to decrease the attack time of the control circuit as the input signal transients become more and more extreme. However, in the absence of a back-up overshoot arrangement, the minimum attack time of the A-type system variable attack time control circuits cannot be shortened sufficiently to sufficiently suppress all overshoots likely to be produced.
Other known variable attack time control circuits have employed control circuits having a fixed attack time control path and a rapid attack time control path, both applied to the device under control. However, a significant drawback to that approach is that the resulting control signal, resulting from the addition of the signals from the control paths, has abrupt variations under transient input conditions causing distortion in the output signal.